9.1 BIORETENTION SYSTEMS

Transcription

1 9.1 BIORETENTION SYSTEMS Bioretention systems are stormwater management facilities used to address the stormwater quality and quantity impacts of land development. The system consists of a soil bed planted with vegetation; it can be underdrained, or runoff can infiltrate into the subsoil. Pollutants are treated through the processes of settling and uptake and filtration by the vegetation. Pollutants are also treated within the soil bed through infiltration. The total suspended solids (TSS) removal rate is 80-90%; this rate will depend on the depth of the soil bed and the type of vegetation selected. N.J.A.C. 7:8 Stormwater Management Rules - Design and Performance Standards Nonstructural Strategies Assist with Strategies #2, 4, 7, and 8; See Page 4 Water Quantity Groundwater Recharge Water Quality Yes, when designed as an on-line system Yes, for systems designed to infiltrate into the subsoil 80-90% TSS Removal, Depending on Vegetation Selection and Depth of Soil Bed Water Quality Mechanisms and Corresponding Criteria Settling Storage Volume Entire Water Quality Design Storm Volume Vegetative Uptake and Filtration Minimum Density of Vegetation 85% Appropriate Species Selection See Chapter 7: Landscaping Depth of Soil Bed feet, See Page 4 Infiltration Maximum Design Storm Drain Time 72 hours, Using Slowest Design Permeability Rate Permeability Rate Factor of Safety 2 Minimum Subsoil Design Permeability Rate Soil Testing Consistent with Appendix E 0.5 inches/hour Required Chapter 9.1 Bioretention Systems Page 1

2 Introduction Bioretention systems are vegetated stormwater management facilities that are used to remove a wide range of pollutants from land development sites; these pollutants include suspended solids, nutrients, metals, hydrocarbons and bacteria. Stormwater runoff entering the system is filtered through the soil bed before discharging downstream through an underdrain or infiltrating into the subsoil. Vegetation in the soil bed provides uptake of pollutants and runoff, and the root system helps maintain the infiltration rate in the soil bed. Bioretention systems may also be used to reduce peak runoff rates when designed as a multi-stage, multi-function facility. In bioretention systems designed to infiltrate into the subsoil, the rate of infiltration is affected by the permeability of the subsoil, the distance separating the basin bottom from the seasonal high water table (SHWT) and the area of the basin bottom. While loss of subsoil permeability through soil compaction is a concern, transport of dissolved pollutants by highly permeable subsoils is of equal concern. Therefore, due to the potential for groundwater contamination, the use of bioretention systems designed to infiltrate into the subsoil is prohibited in areas where high pollutant or sediment loading is anticipated. For more information regarding stormwater runoff that may not be infiltrated, refer to N.J.A.C. 7:8-5.4(a)2.iii. However, this prohibition is limited only to areas onsite where this type of loading is expected; runoff from areas onsite that are grade-separated may be collected in bioretention systems designed to infiltrate into the subsoil provided that the location of the bioretention system is not inconsistent with an NJDEP-approved remedial action work plan or landfill closure plan. Bioretention systems designed to infiltrate into the subsoil may not be used where their installation would create a significant risk of adverse hydraulic impacts. These impacts may include exacerbating a naturally or seasonally high water table so as to cause surficial ponding, flooding of basements, or interference with the proper operation of a subsurface sewage disposal system or other subsurface structure, or where their construction will compact the subsoil. Hydraulic impacts on the groundwater table must be assessed. For more information on groundwater mounding analysis, refer to the USGS Paper on Assessment of Impacts link on the Additional Guidance Documents page at Bioretention systems vary in size and shape. They may be designed in the shape of a traditional basin, or they may be designed in the shape of a swale, making them particularly suited to placement along roadways. Bioretention systems may also be designed with either a flat- or sloped-bottom. This flexibility in design allows bioretention systems to be used in a variety of locations, including lawns, median strips, parking lot islands, vacant lots and some easements. However, they are most effective when they are sited as close as possible to the source of runoff. A bioretention system must have a maintenance plan and, if privately owned, must be protected by easement, deed restriction, ordinance, or other legal measures that prevent its neglect, adverse alteration, or removal. Chapter 9.1 Bioretention Systems Page 2

3 The following illustration shows a street median bioretention system. Runoff is directed into the system through depressed sections of curb; excess runoff flows back to the street where it discharges into the stormwater collection system. Not all bioretention systems may be used for the purposes of meeting the design and performance standards at N.J.A.C. 7:8; only those systems that meet the design criteria presented in this chapter may be used to meet those requirements. Median Bioretention System Plan View Applications The nonstructural stormwater management strategies design standard in the Stormwater Management rules must be addressed for all major developments, pursuant to N.J.A.C. 7:8-5.3(a). The site evaluation for nonstructural strategies should consider all nine strategies. The design of a bioretention system may assist in maximizing the following strategies: Strategy #2: The minimization of impervious surfaces and breaking up or disconnection of the flow of runoff over impervious surfaces. Strategy #4: The minimization of the decrease in time of concentration. Strategy #7: The provision of low maintenance landscaping, which encourages retention, the planting of native vegetation and minimizes the use of lawns, fertilizers and pesticides. Strategy #8: The provision of vegetated, open-channel conveyance. Chapter 9.1 Bioretention Systems Page 3

4 Bioretention systems may be designed to convey storm events larger than the Water Quality Design Storm; however, regardless of the design storm chosen, all bioretention systems must be designed for stability and in accordance with the Standards for Soil Erosion and Sediment Control in New Jersey. Bioretention systems designed to infiltrate into the subsoil may be used to meet the groundwater recharge requirements. For more information on computing groundwater recharge, see Chapter 6: Groundwater Recharge. The depth of the soil bed and the type of vegetation determine the TSS removal rate, as shown in the table below. To receive credit for the selected TSS removal rate, bioretention systems must be designed to treat the Water Quality Design Storm and in accordance with all of the following design criteria. Desired TSS Removal Rate Minimum Depth of Soil Bed Design Parameters Bioretention Vegetation 80% 18 Inches Terrestrial Forested Community 80% 24 inches Site-Tolerant Grasses 90% 24 inches Terrestrial Forested Community Chapter 9.1 Bioretention Systems Page 4

5 Design Criteria Basic Requirements Although the subsurface components differ, underdrained bioretention systems and those designed to infiltrate have common surface elements, which are shown in below. In the following illustrations, a vegetative filter strip provides pretreatment and a downstream collection system receives runoff that does not infiltrate. Bioretention System Basics - Plan View Bioretention System Basics - Profile View Chapter 9.1 Bioretention Systems Page 5

6 The following design criteria apply to both types of bioretention systems. Additional design criteria are found, beginning on page 11, for a system with an underdrain and page 13, for a system designed to infiltrate into the subsoil. Inflow All inflow must be stable and non-erosive and designed in accordance with the Standards for Soil Erosion and Sediment Control in New Jersey. Storage Volume The system must have sufficient storage volume to contain the Water Quality Design Storm runoff volume without overflow. Bioretention systems may be constructed as either off-line or on-line systems. In off-line systems, most, or all, of the runoff from storms larger than the Water Quality Design Storm bypass the bioretention basin through an upstream diversion; this reduces the size of the required basin storage volume, the system s long-term pollutant loading and associated maintenance. On-line systems receive runoff from all storm events; they provide treatment for the Water Quality Design Storm, and they convey the runoff from larger storms through an overflow. These on-line systems store and attenuate the larger storm events and provide runoff quantity control; in such systems, the invert of the lowest quantity control outlet is set at the water surface elevation of the Water Quality Design Storm. The maximum volume that may be infiltrated through the soil bed is the volume generated by the Water Quality Design Storm. All flows in excess of this volume must be discharged through an outlet. For the Water Quality Design Storm, the maximum depth of runoff is 12 inches in a flat-bottom bioretention system and 18 inches at the downstream end of a sloped-bottom system. Infiltration may not be included in routing calculations for quantity control. Infiltration is defined as any discharge of runoff from the system into the subsoil and is sometimes referred to as exfiltration. No standing water may remain at the surface 72 hours after a rain event in order to allow for sufficient storage for the next rain event. Storage times in excess of 72 hours can render a bioretention system ineffective and may result in anaerobic conditions, odor, and both water quality and mosquito breeding issues. Geometry The maximum side slope ratio for earthen embankments is 3:1. The system must have a sufficient surface area to prevent runoff depths in excess of the maximum depth requirement. In sloped-bottom systems, the maximum slope is 10%. Chapter 9.1 Bioretention Systems Page 6

7 Vegetation Bioretention systems are designed with varying wetness zones; therefore, vegetation must be selected and placed based on specific water requirements and tolerances. In general, trees dominate the perimeter zone where there is less frequent inundation, and shrubs and herbaceous species are selected for the zones where inundation is more frequent. The distribution of trees and shrubs must be based on specific site conditions. On average, the number of stems required per acre is 1,000, with trees and shrubs spaced 12 feet and 8 feet apart, respectively. For more information on appropriate vegetation for bioretention systems, see Chapter 7: Landscaping. Soil Bed The soil bed must be a minimum of inches in depth, in accordance with the table on Page 4. The soil bed material must consist of the following mix, by weight: 85 to 95% sand, with no more than 25% of the sand as fine or very fine sands; no more than 15% silt and clay with 2% to 5% clay content. The entire mix must then be amended with 3 to 7% organics, by weight. Pre-mixed soil must be certified to be consistent with the requirement above by either the vendor or by a professional engineer licensed by the State of New Jersey. The content of any soil mixed on-site must be certified by a professional engineer licensed by the State of New Jersey; in addition, the engineer must be present while the soil is mixed. The ph of the soil bed material must range from 5.5 to 6.5. The soil bed material must be placed in lifts not to exceed 8 inches. Additional materials may be necessary to account for settling over time. Safety All bioretention basins must be designed to safely convey overflows to downstream drainage systems. The design of any overflow structure must be sufficient to provide safe, stable discharge of stormwater in the event of an overflow. Safe and stable discharge minimizes the possibility of adverse impacts, including erosion and flooding in down-gradient areas. Therefore, discharge in the event of an overflow must be consistent with the Standards for Off- Site Stability found in the Standards for Soil Erosion and Sediment Control in New Jersey. Bioretention basins that are classified as dams under the NJDEP Dam Safety Standards at N.J.A.C. 7:20 must meet the overflow requirements under these regulations. Overflow capacity can be provided by a hydraulic structure, such as a weir or orifice, or a surface feature, such as a swale or open channel. Chapter 9.1 Bioretention Systems Page 7

8 Outlet Structure For systems designed with an outlet structure, trash racks must be installed at the intake to the outlet structure. They must meet the following criteria illustrated below: Parallel bars with 1-inch spacing between the bars up to the elevation of the Water Quality Design Storm; Parallel bars higher than the elevation of the Water Quality Design Storm must be spaced no greater than one-third the width of the diameter of the orifice or one-third the width of the weir, with minimum spacing between bars of 1 inch and a maximum spacing between the bars of six inches; The trash rack must be designed so as not to adversely affect the hydraulic performance of the outlet pipe or structure; Constructed of rigid, durable and corrosion-resistant material; and Designed to withstand a perpendicular live loading of 300 lbs/sf. An overflow grate is designed to prevent obstruction of the overflow structure. If an outlet structure has an overflow grate, the grate must comply with the following requirements: The overflow grate must be secured to the outlet structure but removable for emergencies and maintenance; The overflow grate spacing must be no greater than 2 inches across the smallest dimension; and The overflow grate must be constructed of rigid, durable, and corrosion resistant material and designed to withstand a perpendicular live loading of 300 lbs./sf. The space below the invert of the discharge pipe must be filled with material, such as concrete, a mixture of sand and cement, or similar grouting material, such that water will not pond in the outlet structure. This material must be sloped towards the discharge pipe to facilitate drainage, as shown on the next page. Chapter 9.1 Bioretention Systems Page 8

9 The minimum diameter of any overflow orifice is 2.5 inches. Blind connections to downstream facilities are prohibited. Any connection to downstream stormwater management facilities must include access points such as inspections ports and manholes, for visual inspection and maintenance, as appropriate, to prevent blockage of flow and ensure operation as intended. All entrance points must adhere to all Federal, State, County and municipal safety standards such as those for confined space entry. In instances where the lowest invert in the outlet or overflow structure is below the flood hazard area design flood or tide elevation in a downstream waterway or stormwater collection system, the effects of tailwater on the hydraulic design of the underdrain and overflow systems, as well as any stormwater quantity control outlets, must be analyzed. Two methods to analyze tailwater are: A simple method entails inputting flood elevations for the 2-, 10-, and 100-year events as static tailwater during routing calculations for each storm event. These flood elevations are either obtained from a Department flood hazard area delineation or a FEMA flood hazard area delineation that includes the 100-year flood elevation or derived using a combination of NRCS hydrologic methodology and a standard step backwater analysis or level pool routing, where applicable. In areas where the 2-year or 10-year flood elevation does not exist in a FEMA or Department delineation, it may be interpolated or extrapolated from the existing data. If this method demonstrates that the requirements of the regulations are met with the tailwater effect, then the design is acceptable. If the analysis shows that the requirements are not met with the tailwater effects, the detailed method below can be used or the BMP must be redesigned. A detailed method entails the calculation of hydrographs for the watercourse during the 2, 10, and 100-year events using NRCS hydrologic methodology. These hydrographs are input into a computer program to calculate rating curves for each event. Those rating curves are then input as a dynamic tailwater during the routing calculations for each of the 2, 10, and 100-year events. This method may be used in all circumstances; however, it may require Chapter 9.1 Bioretention Systems Page 9

10 more advanced computer programs. If this method demonstrates that the requirements of the regulations are met with the tailwater effect, then the design is acceptable. If the analysis shows that the requirements are not met with the tailwater effects, the BMP must be redesigned. Construction Requirements During clearing and grading of the site, measures must be taken to eliminate soil compaction at the location of a proposed bioretention system. The location of the proposed bioretention system must also be cordoned off during construction to prevent compaction of the subsoil by construction equipment or stockpiles. The location of the proposed bioretention system should not be used to provide sediment control during construction; however, when unavoidable, the bottom of the sediment control basin should be at least 2 feet above the final design elevation of the bottom of the soil bed in the bioretention basin. Excavation and construction of a bioretention system must be performed with equipment placed outside the limits of the basin. The excavation to the final design elevation of the bioretention system bottom may only occur after all construction within its drainage area is completed and the drainage area is stabilized. If construction of the bioretention system cannot be delayed, berms must be placed around the perimeter of the system during all phases of construction to divert all flows away from the bioretention system. The berms may not be removed until all construction within the drainage area is completed and the area is stabilized. The contributing drainage area must be completely stabilized prior to bioretention system use. Post-construction testing must be performed on the as-built bioretention system in accordance with the Construction and Post-Construction Oversight and Soil Permeability Testing section in Appendix E: Soil Testing Criteria of this manual. To ensure that the as-built system functions as designed, post-construction testing must include a determination of the permeability rates of the soil bed and the hydraulic capacity of the underdrain, in underdrained systems, or the permeability of the subsoil, in infiltration systems. Where as-built testing results in longer drain times than designed, corrective action must be taken. The drain time is defined as the time it takes to fully infiltrate the maximum design storm runoff volume through the most hydraulically restrictive layer. Access Requirements An access roadway must be included in the design to facilitate monitoring and maintenance. If the access roadway is constructed of impervious material, take note that it may be subject to the stormwater quality, quantity, and/or groundwater recharge requirements at N.J.A.C. 7:8-5.4 and 5.5. Additional steps may be necessary to eliminate vehicular intrusion into the basin footprint, such as from all-terrain vehicles and utility trucks. Chapter 9.1 Bioretention Systems Page 10

11 Types of Bioretention Systems There are two types of bioretention systems: 1. Bioretention Systems with Underdrains 2. Bioretention Systems Designed to Infiltrate into the Subsoil Individual Types of Bioretention Systems The following section provides detailed design criteria for each type of bioretention system; the illustrations include pretreatment provided by a vegetative filter strip. The illustrations show possible configurations and flow paths and are not intended to limit the design. An additional illustration providing a side by side comparison of the two types of bioretention systems is found on page 15. Bioretention Systems with Underdrains Filter fabric is required along the sides and the bottom of the basin to prevent the migration of fine particles from the surrounding soil. The underdrain component consists of three parts the sand layer, the gravel layer and the network of pipes that collect runoff and transport it to the outflow section of the system. The sand layer, which acts as a transition between the soil bed and the subsequent layers, must be at least 6 inches in depth and must consist of clean, medium-aggregate concrete sand (AASHTO M-6/ASTM C-33). To ensure proper system operation, the permeability rate of the sand layer must be at least twice the design permeability rate of the soil bed. The gravel layer must have sufficient depth to provide at least 3 inches of gravel above and below the pipe network and must consist of 0.5 to 1.5 inch clean, broken stone or pea gravel (AASHTO M-43). To ensure proper system operation, the permeability rate of the gravel layer must be at least twice the design permeability rate of the sand layer. Within the gravel layer, the network of pipes, excluding any manifolds and inspection ports, must be perforated. All remaining pipes must be non-perforated. All joints must be secure and watertight. To ensure proper system operation, the network of pipes must have a conveyance rate at least twice as fast as the design flow rate through the sand layer. Inspection ports must be located at the upstream and downstream ends of the perforated section of the network of pipes and extend above the surface of the soil bed. The inspection port exterior must be covered in such a way as to prevent the migration of material into the structure. The depth of runoff generated by the maximum design storm must be marked on all inspection ports and those levels included in the design report and maintenance plan. The capacity of the underdrain must be sufficient to allow the system to drain within 72 hours. The seasonal high water table (SHWT) must be at least 1 foot below the bottom of the gravel layer. Chapter 9.1 Bioretention Systems Page 11

12 All points of access must also be covered in such a way as to prevent sediment or other material from entering the system and to prevent the accumulation of standing water, which could lead to mosquito breeding. The graphic below shows a bioretention system designed with an underdrain. In this illustration, the outlet control structure also serves as the downstream inspection port. Additional maintenance access is provided at the connection to the downstream stormwater collection system. Bioretention System with Underdrain - Profile View Chapter 9.1 Bioretention Systems Page 12

13 Bioretention Systems Designed to Infiltrate into the Subsoil The bottom of a bioretention system must be as level as possible in order to allow runoff to uniformly infiltrate into the subsoil. The SHWT or bedrock must be at least 2 feet below the bottom of the soil bed. The following permeability requirements apply: The permeability of the subsoil must be sufficient to allow the system to drain within 72 hours. Soil tests are required at the exact location of the proposed basin in order to confirm its ability to function as designed. Take note that permits may be required for soil testing in regulated areas, such as areas regulated under the Flood Hazard Area Control Act Rules (N.J.A.C. 7:13), the Freshwater Wetlands Protection Act Rules (N.J.A.C. 7:7A), the Coastal Zone Management Rules (N.J.A.C. 7:7), and the Highlands Water Protection and Planning Rules (N.J.A.C. 7:38). The testing of all permeability rates must be consistent with Appendix E: Soil Testing Criteria in this manual, including the required information to be included in the soil logs, which can be found in section 3.b Soil Logs. In accordance with N.J.A.C. 7:9A-6.2(j)1, Standards for Individual Subsurface Sewage Disposal Systems, the slowest tested permeability must be used for design purposes. Since the actual permeability rate may vary from soil testing results and may decrease over time, a factor of safety of 2 must be applied to the slowest tested permeability rate to determine the design permeability rate. The design permeability rate would then be used to compute the system s drain time for the maximum design volume. The drain time is defined as the time it takes to fully infiltrate the maximum design storm runoff volume through the most hydraulically restrictive layer. The maximum design permeability rate is 10 inches/hour for any tested permeability rate of 20 inches/hour or more. The minimum design permeability rate of the subsoil is 0.5 inches/hour, which equates to a minimum tested permeability rate of 1.0 inch/hour. Filter fabric is required along the sides of the soil bed to prevent the migration of fine particles from the surrounding soil. However, unlike systems with underdrains, filter fabric may not be used along the bottom of the soil bed because it may result in a loss of permeability. As with any infiltration BMP, groundwater mounding impacts must be assessed, as required by N.J.A.C. 7:8-5.4(a)2.iv. This includes an analysis of the reduction in permeability rate when groundwater mounding is present. Where the mounding analysis identifies adverse impacts, the bioretention basin must be redesigned or relocated, as appropriate. The mounding analysis must provide details and supporting documentation on the methods used and assumptions made, including values used in calculations. Chapter 9.1 Bioretention Systems Page 13

14 The illustration below shows a bioretention system designed to infiltrate. Bioretention System with Infiltration - Profile View Chapter 9.1 Bioretention Systems Page 14

15 The following illustration shows the differences between the basic components of a bioretention system with an underdrain and one designed to infiltrate. Cross Section Views A Comparison of the Two Types of Bioretention Systems Chapter 9.1 Bioretention Systems Page 15

16 Designing a Bioretention System The following examples show how to design various bioretention systems to treat the runoff generated by the Water Quality Design Storm. The examples below are two of many possible ways to configure these systems and are not intended to limit the design. Example 1: For the existing two-lane curbed roadway illustrated below, design a bioretention basin to infiltrate the runoff generated by the Water Quality Design Storm. Runoff will enter the proposed system as flow piped from a new shallow catch basin situated along the curb line, and overflow will discharge to the existing stormwater collection system. The following parameters apply: Inflow Drainage Area = 10,000 sf of paved road surface Pavement NRCS Curve Number (CN) = 98 Soil Bed Depth = 18 inches Tested Permeability of Soil Bed = 8 inches/hour Tested Permeability of Subsoil = 2 inches/hour Depth to the Seasonal High Water Table = 8 ft Example 1 Existing Conditions Step #1: Runoff Calculations Using the runoff calculation method described in Technical Release 55 Urban Hydrology for Small Watersheds (TR-55) and discussed in the NRCS Methodology section of Chapter 5: Computing Stormwater Runoff Rates and Volumes, the Water Quality Design Storm runoff volume was calculated to be 862 cf. Step #2: Sizing the Bioretention System In accordance with the design criteria for this type of system, the maximum depth of runoff above the surface of the soil bed is 1 foot. Therefore, the surface area required to infiltrate the Water Quality Design Storm volume of runoff from Step #1 is 862 sf. Chapter 9.1 Bioretention Systems Page 16

17 Step #3: Estimated Drain Time Calculation The drain time is determined by the permeability of the soil bed and the subsoil. The tested permeability of the subsoil is 2 inches/hour, which results in a design permeability of 1 inch/hour. The design permeability of the soil bed is 4 inches/hour; therefore, the permeability of the subsoil is the limiting factor. Because the majority of runoff infiltration occurs through the subsoil, estimation of drain time may only be considered for the basin bottom. For the Water Quality Design Storm, the drain time is calculated as follows: Drain Time = = Water Quality Design Storm Runoff Volume Surface Area x Subsoil Design Permeability Rate 862 cf = 12 hours (862 sf x 1 inch/hour x 1 ft/12 inches) Since this is less than the allowable maximum drain time of 72 hours, the bioretention system appears, at this stage, to meet the drain time requirements. Step #4: Check Separation from SHWT The vertical distance between the lowest elevation of the soil bed and the SHWT must be checked to ensure it meets the minimum requirements. By inspection, 1 ft basin depth ft soil bed depth + 2 ft separation is less than the 8 ft available. Step #5: Groundwater Mounding Analysis Calculate the height of the groundwater mound caused by infiltration to ensure that it will neither prevent infiltration nor damage nearby structures. For information on conducting a groundwater mounding analysis, please see Chapter 6: Groundwater Recharge. For this example, it is assumed the design meets the necessary groundwater mound requirements. Step #6: Overflow Configuration The bioretention system in this example is an on-line system. On-line systems receive runoff from all storms events, and they convey the runoff from larger storms through an overflow, which, in this example, consists of a berm and an overflow riser. The opening in the riser is set at an elevation 1 foot above the surface of the soil bed; this design allows the accumulation of runoff up to the Water Quality Design Storm elevation to infiltrate; excess runoff discharges through the overflow pipe, which is fitted with a debris cap to protect the opening from becoming clogged with vegetative matter and trash. Step #7: Refinements to Design The bioretention system in this example includes earthen embankments. Therefore, the overall size of the system will have to account for the required 3:1 minimum side slopes. As-built testing must be conducted to validate the assumptions in the problem statement, confirm the design permeability rate of the subsoil and memorialize the design drain time of the system in the maintenance plan. Chapter 9.1 Bioretention Systems Page 17

18 The following illustrations show this bioretention system in plan, profile and cross sectional views. Take note that the horizontal distance between the inflow and outflow riser allows for maximum contact time between the runoff and the vegetation, which promotes greater pollutant removal. Example 1 - Plan View Example 1 - Profile and Cross-Section View Chapter 9.1 Bioretention Systems Page 18

19 Example 2: For the existing uncurbed section of roadway illustrated below, design a bioretention swale with an underdrain to treat the runoff generated by the Water Quality Design Storm. Runoff will enter the proposed system as overland flow from the adjacent road surface. Additionally, a connection to the existing downstream stormwater collection system will be required in order to maintain safe travel conditions. The following parameters apply: Inflow Drainage Area = 5,000 sf of paved road surface Pavement NRCS Curve Number (CN) = 98 Soil Bed Depth = 18 inches Assumed Design Permeability of Soil Bed = 4 inches/hour Water Quality Design Storm Depth = 18 inches = 1.5 feet Example 2 Existing Conditions Step #1: Runoff Calculations Using the runoff calculation method described in Technical Release 55 Urban Hydrology for Small Watersheds (TR-55) and discussed in the NRCS Methodology section of Chapter 5: Computing Stormwater Runoff Rates and Volumes, the Water Quality Design Storm runoff volume was calculated to be 431 cf. Step #2: Preliminary Shape and Size of the Bioretention Swale For an initial approximation, a simple shape is selected. If the swale is represented by a wedge shape, the surface area of the soil bed can be estimated by dividing the runoff volume from Step #1 by the average depth of runoff in the swale. For the Water Quality Design Storm, the upstream depth is set at zero, and the surface area is calculated as follows: Surface Area = 431 cf (0.5 x 1.5 ft) = 575 sf However, the swale cross-section cannot be rectangular. That would create a sharp drop-off adjacent to the roadway, resulting in erosion of the soil bed. The nature of the flow as it enters the bioretention swale, as well as the nature of future maintenance tasks that will be required, must be taken into account when designing a bioretention swale. A more complex shape must be evaluated and checked for compliance with the minimum SHWT separation requirement; therefore, the assumed shape is a flat-bottom swale with sloping sides. In this configuration, the longitudinal slope directs runoff toward the downstream end, and the collected runoff Chapter 9.1 Bioretention Systems Page 19

20 forms a prismatic shape with a trapezoidal downstream face. This shape can be thought of as a central wedge flanked by two symmetrical triangular pyramids, shown in the following illustration. The ends of the two pyramids are shaded in yellow and that of the central wedge in white in the cross-section view. Example 2: Geometry Runoff will occupy not only the footprint of the swale, shown in pink in the plan view portion of the above illustration, but also the two side pyramids shown in blue. Failure to account for this additional volume results in both an oversized swale and the infiltration, during larger storm events, of more volume than allowed. Calculating the volume of this complex shape by hand, although possible, is beyond the scope of this chapter and is easily performed by computer programs. Step #3: Estimated Drain Time Calculation Since there is no infiltration into the subsoil, the limiting factor in the drain time calculation is the permeability rate of the soil bed. If the method employed in Step #3 of Example 1 was followed, the assumed soil bed design permeability and the footprint area shaded above in pink would determine the drain time. Following Example 1, an estimate of the drain time for the Water Quality Design Storm would be calculated as follows: Drain Time = Water Quality Design Storm Runoff Volume Footprint Area x Soil Bed Design Permeability Rate However, the above method cannot be used because the swale has a sloped bottom, meaning the area available for infiltration will vary with time as the water level decreases. The area available for infiltration that is present at any given moment is a function of the depth of the runoff in the swale at that moment. The drain time calculation could be written as a summation of all the incremental volumes divided by the soil bed permeability rate, but in the end, the maximum design depth governs the calculation. The drain time estimate is therefore as follows: Chapter 9.1 Bioretention Systems Page 20

21 Drain Time = Maximum Runoff Depth Soil Bed Design Permeability Rate = 18 inches = 4.5 hours 4 inches/hour Since this is less than the allowable maximum drain time of 72 hours, the bioretention system appears, at this stage, to be sized correctly to meet the drain time requirements. Step #4: Overflow Configuration The bioretention swale in this example is an on-line system. On-line systems receive runoff from all storms events, and they convey the runoff from larger storms through an overflow, which, in this example, consists of a berm and an overflow riser. The opening in the riser is set at an elevation 1.5 feet above the surface of the soil bed; this design allows the accumulation of runoff up to the Water Quality Design Storm elevation to infiltrate; excess runoff discharges through the overflow pipe, which is fitted with a debris cap to protect the opening from becoming clogged with vegetative matter and trash. Step #5: Underdrain Design To ensure that the underdrain does not provide the hydraulic control of the system, the pipe network must be designed with conveyance rates at least twice as fast as the design flow rate through the sand layer. Additionally, the pipes must be sloped for complete drainage. The required clearances within the gravel layer must also be provided. Step #6: Check Separation from SHWT The vertical distance between the lowest elevation of the gravel layer and the SHWT must be checked to ensure it meets the minimum 1 ft separation requirement. Step #7: Refinements to the Design The overall size of the system will have to account for the two triangular pyramids on the sides which have a 3:1 slope. Additionally, rip-rap is placed between the bioretention swale and the edge of pavement to prevent erosion. Prior to installation of the soil bed, permeability testing is not feasible; therefore, for design calculations, a permeability rate of 4 inches/hour was assumed. This assumed permeability rate included the required factory of safety, which translates to an assumed tested permeability rate of 8 inches/hour. As-built testing must be conducted to validate this assumption and establish the design drain time of the system, which must also be included in the maintenance plan. The illustrations on the following page show this bioretention swale in plan, profile and cross sectional views. In this example, an additional vertical space above the outflow riser, although not required, is included. This additional vertical space is intended to ensure that the swale does not flood the roadway in the event that debris partially clogs the cap on the overflow riser. This additional space does not increase the volume of runoff infiltrated, as the opening in the outflow riser directs excess runoff to the downstream collection system. The overall size of the swale in this example includes end berms as transition areas to the existing grade elevation to account for this additional depth. Chapter 9.1 Bioretention Systems Page 21

22 Example 2 - Plan View Example 2 - Profile and Cross-Section View Chapter 9.1 Bioretention Systems Page 22

23 Considerations When planning a bioretention system designed to infiltrate into the subsoil, consideration should be given to soil characteristics, depth to the groundwater table, sensitivity of the region, and inflow water quality. It is also important to note that the use of bioretention systems designed to infiltrate into the subsoil is recommended in this manual only for the Water Quality Design Storm or smaller storm events. Use of these basins to infiltrate larger volumes, should only be considered when another applicable rule or regulation requires the infiltration of a larger storm event. In such a case, the bioretention system should be designed to infiltrate the minimum storm event required to address that rule or regulation. In addition to the prohibition of recharge in the areas with high pollutant loading or with runoff exposed to source material as defined in N.J.A.C. 7:8-5.4(a)2iii, the utilization of bioretention basins should consider the impact of infiltration on subsurface sewage disposal systems, water supply wells, groundwater recharge areas protected under the Ground Water Quality Standards rules at N.J.A.C 7:9C, streams under antidegradation protection by the Surface Water Quality Standards rules at N.J.A.C. 7:9B, or similar facilities or areas geologically and ecologically sensitive to pollutants or hydrological changes. Furthermore, the location and minimum distance of the bioretention basin from other facilities or systems shall also comply with all applicable laws and rules adopted by Federal, State, and local government entities. Pretreatment As with all other best management practices, pretreatment may extend the functional life and increase the pollutant removal capability of a bioretention system by reducing incoming velocities and capturing coarser sediments. Pretreatment may consist of a forebay or any of the structural BMPs found in Chapter 9: Structural Stormwater Management Measures. There is no adopted TSS removal rate associated with forebays; therefore, their inclusion in any design should be solely for the purpose of facilitating maintenance. Forebays may be earthen, constructed of riprap, or made of concrete and must comply with the following requirements: The forebay must be designed to prevent scour of the receiving basin by outflow from the forebay. The forebay should provide a minimum storage volume of 10% of the Water Quality Design Storm and be sized to hold the sediment volume expected between clean-outs. The forebay should fully drain within nine hours in order to facilitate maintenance and to prevent mosquito issues. Under no circumstances should there be any standing water in the forebay 72 hours after a precipitation event. Surface forebays must meet or exceed the sizing for preformed scour holes in the Standard for Conduit Outlet Protection in the Standards for Soil Erosion and Sediment Control in New Jersey for a surface forebay. If a concrete forebay is utilized, it must have at least two weep holes to facilitate low level drainage. Chapter 9.1 Bioretention Systems Page 23

24 When using a structural BMP for pretreatment, it must be designed in accordance with the design requirements outlined in its respective chapter. For additional information on the design requirements of each structural BMP, refer to the appropriate chapter in this manual. Mulch Layer The mulch layer on the surface of the soil bed may enhance the performance of the bioretention system. Mulch can aid in plant growth by retaining moisture and by providing an environment for microorganisms that decompose incoming organic matter. Additionally, the mulch layer can act as a filter for finer particles in runoff preventing these particles from clogging the soil bed. Care must be taken to ensure that the mulch layer does not reduce the design permeability rate of the surface. The mulch layer should consist of standard 1- to 2-inch shredded hardwood or chips. The mulch layer should be 2 to 4 inches in depth and replenished as necessary. To determine whether a mulch layer is appropriate for on-line systems, consideration must be given to issues such as scour and floatation of the mulch during large storm events. Enhancing Pollutant Removal Maximizing the horizontal distance between inflow and overflow structures in a bioretention system may increase contact time for stormwater runoff with the system vegetation and soil bed, where pollutant-removing chemical and biological processes occur. Grouping inflows near outflows may locally decrease the capacity of the soil bed to remove metals and other dissolved nutrients as well as the ability of the plants to uptake pollutants. Increasing the soil bed depth to inches may enhance pollutant removal and accommodate more deeply-rooted plants. Soil Characteristics For bioretention systems designed to infiltrate into the subsoil, soils are perhaps the most important consideration for site suitability. In general, County Soil Surveys may be used to obtain necessary soil data for planning and preliminary design of bioretention systems. However, as previously mentioned, for final design and construction, soil tests are required at the exact location of the proposed system in order to confirm its ability to function properly without failure. In order to confirm reasonable data consistency, the results of soil testing should be compared with the County Soil Survey data that was used in the computation of runoff rates and volumes and the design of on-site BMPs. If significant differences exist between the soil test results and the County Soil Survey data, additional soil tests are recommended to determine and evaluate the extent of the data inconsistency and whether there is a need for revised site runoff and BMP design computations. All significant inconsistencies should be discussed with the local Soil Conservation District prior to proceeding with such a redesign to help ensure that the final site soil data is accurate. Chapter 9.1 Bioretention Systems Page 24

25 Geology The presence or absence of Karst topography is an important consideration when designing a bioretention system designed to infiltrate into the subsoil; in areas of the State with this type of geology, the bedrock is composed of highly soluble rock. If Karst topography is present, infiltration of runoff may lead to subsidence and sinkholes; therefore, only bioretention systems designed with underdrains should be used in these areas. For more information on design and remediation in areas of Karst topography, refer to the Standards for Soil Erosion and Sediment Control in New Jersey: Investigation, Design and Remedial Measures for Areas Underlain by Cavernous Limestone. Maintenance Regular and effective maintenance is crucial to ensure effective bioretention system performance; in addition, maintenance plans are required for all stormwater management facilities on a major development. There are a number of required elements in all maintenance plans, pursuant to N.J.A.C. 7:8-5.8; these are discussed in more detail in Chapter 8: Maintenance of Stormwater Management Measures. Furthermore, maintenance activities are required through various regulations, including the New Jersey Pollutant Discharge Elimination System (NJPDES) rules, N.J.A.C. 7:14A. Specific maintenance requirements for bioretention systems are presented below; these requirements must be included in the maintenance plan. General Maintenance Proper and timely maintenance is essential to continuous, effective operation; therefore, an access route must be incorporated into the design, and it must be properly maintained. All structural components must be inspected, at least once annually, for cracking, subsidence, spalling, erosion and deterioration. Components expected to receive and/or trap debris and sediment must be inspected for clogging at least four times annually, as well as after every storm exceeding 1 inch of rainfall. Sediment removal should take place when all runoff has drained from the planting bed and the basin is dry. Disposal of debris, trash, sediment and other waste material must be done at suitable disposal/recycling sites and in compliance with all applicable local, state and federal waste regulations. In systems with underdrains, the underdrain piping must be connected, in a manner that is easily accessible for inspection and maintenance, to a downstream location. Access points for maintenance are required on all enclosed areas within a bioretention system; these access points must be clearly identified in the maintenance plan. In addition, any special training required for maintenance personnel to perform specific tasks, such as confined space entry, must be included in the plan. Chapter 9.1 Bioretention Systems Page 25

26 Stormwater BMPs may not be used for stockpiling of plowed snow and ice, compost, or any other material. Vegetated Areas Bi-weekly inspections are required when establishing/restoring vegetation. A minimum of one inspection during the growing season and one inspection during the nongrowing season is required to ensure the health, density and diversity of the vegetation. Mowing/trimming of vegetation must be performed on a regular schedule based on specific site conditions; perimeter grass should be mowed at least once a month during the growing season. Grasses within the bioretention system must be carefully maintained with lightweight equipment, such as a hand-held line trimmer, in order to maintain the permeability of the system. Vegetative cover must be maintained at 85%; damage in excess of 50% must be addressed through replanting in accordance with the original specifications. Vegetated areas must be inspected at least once annually for erosion, scour and unwanted growth; any unwanted growth should be removed with minimum disruption to the remaining vegetation. All use of fertilizers, pesticides, mechanical treatments and other means to ensure optimum vegetation health must not compromise the intended purpose of the bioretention system. Drain Time The planting bed must be inspected at least twice annually to determine if the permeability of the bed has decreased. The design drain time for the maximum design storm runoff volume must be indicated in the maintenance manual. If the actual drain time is longer than the design drain time, the components must be evaluated and appropriate measures taken to return the bioretention system to the original tested as-built condition. If the bioretention system fails to drain the Water Quality Design Storm within 72 hours, corrective action must be taken and the maintenance manual revised accordingly to prevent similar failures in the future. Chapter 9.1 Bioretention Systems Page 26

27 References Claytor, R. and T. Schueler. December Design of Stormwater Filtering Systems. The Center for Watershed Protection. Ellicott City, MD. Hsieh, C. and A. Davis. November Evaluation and Optimization of Bioretention Media for Treatment of Urban Stormwater Runoff, Journal of Environmental Engineering, American Society of Civil Engineers. New York, NY. Livingston, E.H., H.E. Shaver, J.J. Skupien and R.R. Horner. August Operation, Maintenance, & Management of Stormwater Management Systems. In cooperation with U.S. Environmental Protection Agency. Watershed Management Institute. Crawfordville, FL. Lucas, William C. March Draft Green Technology: The Delaware Urban Runoff Management Approach. TRC Omni Environmental Corporation. Princeton, NJ. Maryland Department of the Environment Maryland Stormwater Design Manual Volume 1 Stormwater Management Criteria. Water Management Administration. Baltimore, MD. New Jersey Department of Agriculture. January Standards for Soil Erosion and Sediment Control in New Jersey. State Soil Conservation Committee. Trenton, NJ. New Jersey Pinelands Commission. September Pinelands Comprehensive Management Plan. New Lisbon, NJ. North Carolina State University Cooperative Extension Urban Waterways: Bioretention Performance, Design, Construction and Maintenance. Raleigh, NC. Ocean County Planning and Engineering Departments and Killam Associates. June Stormwater Management Facilities Maintenance Manual. New Jersey Department of Environmental Protection. Trenton, NJ. Pennsylvania Association of Conservation Districts and Pennsylvania Department of Environmental Protection Pennsylvania Handbook of Best Management Practices for Developing Areas. Natural Resources Conservation Service. Harrisburg, PA. Chapter 9.1 Bioretention Systems Page 27